1. Field of the Invention
The present invention relates to an improved process for electroplating a metal strip. It is particularly applicable to the formation of a zinc coating or deposit on a strip.
2. Discussion of the Background
It is known how to form a zinc coating on a strip by making the latter run through a bath of at least one electrolytic cell, in which the strip constitutes the cathode. This cell is fed with an adjustable current. The conditions for forming the zinc deposit depend in particular on the current density at the surface of the strip. There is a limiting value of the current density, called the "limiting diffusion current of the zinc ions", for which the deposit crystallises in a dendritic form whose macroscopic appearance is called "burnt deposit" by the person skilled in the art. Furthermore, at such values of current density, there occurs a release of hydrogen, part of which may be adsorbed by the strip. This release of hydrogen may lead to a formation of zinc hydroxide, Zn(OH).sub.2, by local modification of the pH, which is fixed between the strip and the zinc deposit. The operations performed subsequently on the strip, such as painting, can then lead to a degradation of the strip.
The current feeding the electrolytic cell is normally adjusted so as to obtain, at the surface of the strip, a current density less than the aforementioned limiting value. Prior to the electroplating operation, operations for preparing the strip are performed, especially comprising a degreasing operation. However, despite these preparation operations, impurities sometimes remain at the surface of the strip causing the deviation of the current lines and the appearance of overcurrent zones around these impurities. In these zones, the current density can reach, or indeed exceed, the limiting diffusion current, thereby causing the phenomenon mentioned hereinabove of adsorption of hydrogen at and of fixing of zinc hydroxide to the surface of the strip.
In this case, in addition to the reduction in quality of the adherence of the zinc coating, a microscopic loss of cohesion of the coating is observed when the strip subsequently undergoes a stoving treatment intended, for example, to polymerise a paint layer covering the coated strip. Hydrogen, which has been adsorbed at the surface of the strip during the electroplating operation, is released during the stoving, thereby forming craters and pimples at the surface of the coating of the strip.